1. Field of the Invention
The present invention relates to nondestructive testing of paper for mechanical properties. More particularly, the present invention relates an apparatus for continuously testing a paper web ultrasonically while traveling within the papermaking machine.
2. Prior Art
Paper and paperboard strength properties are important to most converting and end-use applications. Mechanical parameters such as ultimate tensile strength, burst, and bending stiffness are the strength indicia of greatest concern to the papermaker.
Tests to ascertain these mechanical characteristics of a given paper web have, traditionally, been destructive of the test sample or specimen. Loss of the sample is of no consequence but the quantity of specialized test equipment, skills and time required to perform the full battery of such tests is enormous when it is considered that a reliable test average requires a large number of test samples.
Over the past decade, nondestructive ultrasonic methods have been developed to measure many of the mechanical properties previously measured by destructive tests. By these methods, a vibration induced sonic disturbance is transmitted through a sample sheet and the resultant wave velocity is measured. Such velocity measurements are taken relative to the test sample fiber orientation, both MD (machine direction) and CD (cross-machine direction), and used to calculate the sheet in-plane elastic parameters of Young's and shear moduli.
As published by Tappi, Vol. 48, No. 3, March, 1965, the technical paper titled "Nondestructive Sonic Measurement of Paper Elasticity" by J. K. Craver and D. L. Taylor thoroughly develops the theoretical relationships between sonic wave velocity and a paper web modulus of elasticity. From the basic theory of Craver and Taylor, a number of authors contributed to the evolution of theory and technology embodied by the report of G. A. Baum and C. C. Habeger concerning "On-line Measurement of Paper Mechanical Properties" appearing in Tappi, Vol. 63, No. 7, July, 1980. U.S. Pat. No. 4,291,577 issued Sept. 29, 1981 to G. A. Baum and C. C. Habeger, discloses the substance of the authors' earlier Tappi report and describes a mechanical system for measuring the sonic wave propagation velocity within a traveling paper web as found within the papermaking machine. Pursuant to the system of Baum and Habeger, three synchronously coordinated idler wheels are positioned in friction drive contact with a moving paper web. Each of the wheels include a single piezoelectric transducer located on the wheel rim for cyclic contact with the traveling web surface. Wheel rotation is mechanically timed so that all three transducers are in simultaneous contact with the paper web. One of the three transducers serves as an ultrasonic signal transmitter whereas the other two are receivers. Relative to the transmitter wheel, one receiver is located at a known distance along the web in the machine direction and the other at the same known distance in the cross-machine direction. When the transducers engage the web, an electrical signal to the transmission transducer stimulates an ultrasonic mechanical vibration which is transmitted through the paper web to the receiver transducers. Responsively, the receivers emit electrical signals for receipt by electronic data process equipment which compares and determines the time interval between original signal emission and signal receipt. This approach to wave velocity measurement has been characterized as the "time-of-flight" technique.
Also applied to paper measurement has been the "wave-phase shift" technique. By the wave-phase shift method, the web contacting wheels are constructed with four quadrant arc piezoelectric transducers for substantially continuous contact with the paper web. A continuously emitted 6.75 kH.sub.z sine signal, for example, is electronically processed for phase shift determination. From the phase shift, the wave velocity is derived. The characterization as a "wave-phase shift" measurement technique distinguishes this measurement method from the "time-of-flight" technique.
Both of the prior art wave velocity measurement techniques have respective advantages and disadvantages. The time spans between signal stimulation and receipt are extremely short: in the microsecond range. It is difficult to reliably segregate signal pulses or phase displacements of such short duration from extraneous noise always present in a moving paper web.
Although the "wave-phase shift" signals are continuous and therefore avoid the uncertanties of mechanically timed pulsing intervals, other signal obfuscation mechanisms are operative. For example, each of the piezoelectric transducers are original signal generators of continuous, low amplitude signals over a wide frequency spectrum and relatively high amplitude signals at a natural frequency determined by the momentary web speed driving the transducer wheels. Simultaneously, the transducers respond to sympathetic and harmonic vibrations originating from other sources around the paper machine but not directly related to the machine operation. Even the measurement signal, when received by a reception transducer through a carriage conduit other than a direct route through the web mass, is a noise source due to a greater or less transmission interval depending on the transmission route.
In the midst of all these spurious signal sources, it is essential to instrument reliability that only the original stimulation signal directly transmitted through the web mass is the signal processed for sonic velocity measurement. It is, therefore, a first object of the present invention to teach a process and apparatus for distinguishing these desired, web transmitted, acoustic signals from irrelevant noise signals.
Another object of the present invention is to teach the construction of a novel and acoustically quiet transducer assembly for measuring paper web strength properties.